The invention is related to implantable medical leads, and more particularly to implantable cortical electrical leads used to sense electrographic signals from a patient""s brain or to apply electrical stimulation to the brain.
In the medical diagnosis and treatment of various brain disorders, including epilepsy, Parkinson""s disease, sleep disorders, and psychiatric ailments, it is customary and frequently useful to analyze electrical signals originating in the brain. For a review of this technology, see Ajmone-Marsan, C., Electrocorticography: Historical Comments on its Development and the Evolution of its Practical Applications, Electroencephalogr. Clin. Neurophysiol. Suppl. 1998, 48:10-16; there are numerous other applications. These electrographic signals are commonly known as electroencephalogram (EEG) signals when originating or received at the surface of the brain, such as from scalp electrodes, or electrocorticogram (ECoG) signals when originating or received below the surface of the brain, such as from intracranial electrodes. The term xe2x80x9cEEGxe2x80x9d will be used generically herein to refer to both types of signals.
It is also becoming accepted to apply electrical stimulation to various structures of the brain for both diagnostic and therapeutic purposes. For an exemplary diagnostic application, see Black, P. M. and Ronner S. F., Cortical Mapping for Defining the Limits of Tumor Resection, Neurosurgery 1987, 20:914-919, which addresses the use of electrical stimulation via deep brain electrodes to identify functional portions of the brain prior to and as a planning stage in surgical resection. For an example of a therapeutic application, see Cooper, I. S. and Upton, A. R. M., Effects of Cerebellar Stimulation on Epilepsy, the EEG and Cerebral Palsy in Man, Electroencephalogr. Clin. Neurophysiol. Suppl. 1978, 34:349-354. In both of these examples, acutely implanted brain electrodes are connected to external equipment.
It is also contemplated that chronic electrical stimulation can be used as a direct treatment for disorders such as epilepsy. See, e.g., U.S. Pat. No. 6,016,449 to Fischell, et al., which describes an implantable neurostimulator that is coupled to relatively permanent deep brain electrodes.
Although it is frequently possible to employ scalp electrodes for certain types of EEG monitoring and analysis, it has been found that ambient electrical noise (such as from the 50/60 Hz power system) can adversely impact signal-to-noise ratio, and certain signal components of interest may be filtered out by the patient""s intervening cranium and scalp tissue. Moreover, precise localization is less feasible with scalp electrodes.
Accordingly, intracranial signal analysis, that is, the consideration of signals that originate from within a patient""s cranium, whether by internal or external apparatus, is best accomplished with brain surface electrodes, such as strip and grid electrodes, cortical depth leads, or some combination of surface electrodes and depth leads.
Typical brain surface strip and grid electrode arrays consist of flat, disk-shaped electrodes that are placed on the surface of the patient""s brain. In a typical strip or grid electrode array, each electrode has an exposed diameter of approximately 3 mm (or xe2x85x9 inch), and the electrodes are distributed along a line (for a strip electrode array) or in a rectangular grid (for a grid electrode array) at a pitch of approximately 10 mm.
Unfortunately, brain surface strip and grid electrode arrays have a tendency, particularly with long-term chronic use, to move away from the surface of the brain. This can be caused by atrophy or other mechanisms associated with cerebrospinal fluid (CSF) dynamics. The result is frequently unsatisfactory or intermittent electrical contact between the electrodes and the desired brain tissue. It frequently requires further surgery (with the associated risks for the patient) or electronic compensation for the change in characteristics (with a potentially harmful increase in stimulation current being delivered to the brain, or at minimum, decreased signal-to-noise ratio), and may result in long-term performance deterioration. There is no known acceptable way to anchor a traditional strip or grid electrode array to the surface of the brain. While the electrode may be anchored to the patient""s cranium or dura mater, the brain tends to recede from these structures in certain cases. Moreover, the electrodes are spaced evenly along a line or grid, and while it is possible to orient a strip or grid electrode array in a desired manner, it is generally not possible to position the individual electrodes independently.
Typical brain depth leads are flexible small-diameter (usually 1-1.5 mm) round leads having distal electrodes. It is known for depth leads to have multiple independent distal electrodes on the same lead shaft, but such electrodes are generally located coaxially along a distal portion of the shaft. It is difficult, and usually impractical, to attempt to position the individual electrodes independently.
Accordingly, it would be desirable to have an implantable medical electrical lead that provides the advantages of both surface electrodes and depth leads, along with other advantages. Such an electrical lead would have multiple distal electrodes that are independently positionable near the surface of the brain or in deep brain structures, and would remain in contact with the desired neural tissue regardless of atrophy or other adverse conditions.
A medical electrical lead in accordance with the present invention has a furcated, or split, distal portion with two or more separate end segments, each of which bears at least one sensing or stimulation electrode. The end segments are individually positionable and narrow in diameter, and in various configurations may be used as depth leads or positioned near the surface of the brain to operate in a manner similar to surface electrodes. In either case, the distal tips of the end segments are anchored in brain tissue, either in deep brain structures or near the surface, as desired by the treating practitioner. Accordingly, in comparison to traditional strip and grid electrode arrays, there is less likelihood that atrophy or other physiological mechanisms will cause the electrodes to dislodge from the cortex.
A furcated lead according to the invention is preferably provided with an inline lead connector area at its proximal end, to maintain compatibility with known, existing, and improved inline lead connectors. Inline lead connectors tend to be the smallest and easiest to operate type of lead connectors, both important characteristics, especially in the surgical and implantable arenas.
With leads in accordance with the present invention, it is possible to realize several additional advantages. With plural individually positionable distal electrodes, it is possible to reach a larger number of separate brain sites while limiting the number of leads necessary to do so. The number of leads connected to an external apparatus or implanted neurostimulator (or other device) is minimized, thereby improving the ease of treating the patient, improving ease of lead management, reducing the possibility of lead breakage, and reducing the possibility of discomfort or erosion under the patient""s scalp.